Field of the Invention
This invention relates to methods of, and apparatus for, growing crystalline material; and more particularly, to a method affording real-time analysis and control of melt-chemistry in crystal growing operations.
It is conventional to grow a single-crystalline ingot from a polycrystalline material by preparing a melt of such material and contacting the surface of the melt with a previously prepared seed crystal of the same material but with the desired crystalline lattice orientation. In growing the single crystalline ingot, the seed crystal is withdrawn from the melt at a rate of the order of a few inches per hour while the seed crystal (and hence the ingot), is counter-rotated with respect to the melt. With this technique, single crystalline ingots several feed in length and several inches in diameter are routinely grown, particularly in the silicon semiconductor industry.
In this industry, it is conventional to dope the melt with either an N-type dopant impurity, such as phosphorus, antimony, or arsenic, and/or a P-type dopant impurity, such as boron, aluminum and gallium.
A serious problem facing this industry is the difficulty of controlling the dopant concentration of the melt and the ultimately grown single crystalline ingot, and hence the resistivity of such ingot. There are many reasons why it is desirable to control the resistivity of the grown crystalline ingot. For example, control of resistivity is required for the preparation of electrically isolated portions of wafers which have been cut from the ingot in the manufacture of integrated circuits. Also, since resistivity has a bearing on the depth to which dopant impurities may be diffused and a bearing on the concentration gradient of diffused dopant impurities, it is necessary to control resistivity. Often the manufacture of certain semiconductor devices requires the control of resistivity of wafers used in making the devices to within narrow, and difficult to achieve, ranges.
One reason for the difficulty of controlling resistivity is that while growing a crystalline ingot from a melt, the chemistry of the melt (i.e., the concentration of dopant impurity with respect to the basic material, for example silicon) does not remain constant as the growing operation, which may require many hours, proceeds. Rather, the dopant impurity tends to evaporate from the melt at a rate depending in complex ways upon temperature, temperature gradients, geometry, concentration, and vapor pressure in the crystal growing apparatus.
Another reason for the difficulty of controlling the resistivity is segregation effects, whereby the concentration of the dopant impurity which becomes a part of the grown crystalline ingot is not the same as the concentration of the dopant impurity in the melt itself. More specifically, the concentration of the dopant impurity is usually less in the grown crystalline ingot than in the melt. As a result, the dopant concentration within the ingot itself increases with longitudinal position in a complex way which is not readily predictable precisely from prior empirical results.
The evaporation and segregation effects previously mentioned are particularly troublesome for melts which have been recharged several times. This recharging involves growing a single crystalline ingot of less than full length and width from a polycrystalline melt, adding new polycrystalline material to the melt, melting such material, and continuing to grow the single crystalline ingot.
Moreover, it is advantageous to reconstitute a melt after a crystalline ingot has been grown by adding more polycrystalline material. Thus, avoidable is the costly and time-consuming process of cooling the remaining melt and completely restarting the system with a new charge of polycrystalline material and dopant impurity, and achievable is uniformity of resistivity among the successively grown ingots. From this aspect, it is desirable in general to analyze a melt from which a crystalline ingot has been grown, or while a crystalline ingot is being grown, and to recharge that melt with a proper amount of polycrystalline material and/or dopant impurity so that second and successive crystalline ingots can be grown without cooling down the system with a consequent substantial saving in electrical energy, polycrystalline material and crucibles for the melt.
It is highly advantageous for ecological and economic reasons to recycle various portions of grown crystalline ingots which are not suitable for other uses for whatever reasons. Such portions may include the ends of grown crystalline ingots, as well as junk which may be broken or otherwise deleteriously affected in subsequent processing. Because such material, which may be referred to as primary material, has various concentrations of dopant impurities, it is difficult to control the resistivity of the grown crystalline ingot. Using real-time analysis and control of melt-chemistry in a crystal growing operation, a quantity of material of undetermined chemistry can be melted, analyzed, and, by adding amounts of primary material and/or dopant impurity, the melt-chemistry can be adjusted to a desired composition known to be suitable for growing a crystalline ingot of a predetermined desired resistivity.
Still another reason for the difficulty of controlling the resistivity of the grown crystalline ingot are the random sources of dopant and other impurities that increase the impurities beyond the desired amount. Such impurities are phosphorus and/or boron which are often contained in the primary material or the crucible for holding the melt.